The present invention relates to production and purification of diborane. Diborane (B.sub.2 H.sub.6) is a flammable gas which is used as a p type dopant in semiconductors, and is also used in boron-phosphate-silicate glass forming. Diborane forms a wide variety of complexes with lewis bases such as boranetetrahydrofuran, borane dimethyl sulfide and a variety of amine boranes. These compounds are widely used as selective reducing agents in synthesis of pharmaceuticals, fine organic chemicals and electroless metal plating baths.
At room temperature, diborane slowly decomposes to higher boranes with their physical state ranging from gaseous to solid. This causes process variations and equipment malfunctions. In order to reduce decomposition, diborane is sometimes shipped as a mixture with a blanket gas or at low temperature, such as at dry ice temperature. Another way to overcome the decomposition problem is to employ point-of-use diborane generation. However, the difficulties encountered with present synthesis and purification processes have inhibited point-of-use diborane generation.
Numerous possible methods of diborane synthesis have been published. The most typical and commercially used synthesis method is the reaction of sodium borohydride with boron trifluoride in ether solvents such as diglyme. Because this process uses highly inflammable solvents, it requires significant safety precautions. Further, diborane complexes with solvents. Such complexes make it difficult to purify the diborane.
A preferred dry process for diborane synthesis is described in U.S. Pat. No. 4,388,284. This process involves reaction of lithium or sodium borohydride with boron trifluoride (BF.sub.3) in the absence of a solvent. As a preferred method, the patent describes condensing gaseous boron trifluoride at liquid nitrogen temperature onto sodium borohydride, then warming the resultant mixture to a reaction temperature of 0 to 50.degree. C. and holding the mixture at the reaction temperature for 4 to 12 hours. The process yields a mixture containing about 95% diborane and also containing unreacted boron trifluoride. Under similar conditions, reaction of lithium borohydride with boron trifluoride is sluggish and gives poor yield.
While the dry process provides diborane free from solvent contamination, the product contains significant amount of unreacted boron trifluoride. To achieve high purity diborane, tedious distillation is required to separate the diborane from the boron trifluoride. The process is slow for commercial production and is a batch process. Based upon thermodynamic considerations, the reaction of lithium borohydride with boron trifluoride should be more favored than the comparable reaction with sodium borohydride, but the observations set forth in the '284 patent indicate that the reaction involving lithium borohydride does not work well in practice.